2003ApJ...588L.105P

We present simultaneous VLBI maps of the J=1⟶0 (43.1 GHz) and J=2⟶1 (86.2 GHz) silicon monoxide (SiO) v=1 masers in the circumstellar envelope of the asymptotic giant branch star R Cassiopeiae that have been registered to submilliarcsecond precision. Our images allow specific comparisons of maser strengths from volumes that are about 1/10 AU in projected size. Bright 86.2 GHz v=1 masers in two cases operate from the same gas volumes as the 43.1 GHz v=1 masers. The relative strengths of the 43.1 and 86.2 GHz masers vary from roughly unity to lower limits of several hundred to 1. The low strength of J=2⟶1 masers relative to J=1⟶0 might arise from neutral densities of H₂≤4x10⁹ cm^–3 at 1.5-2 times the photospheric radius from R Cas. The SiO masers of R Cas at this epoch are not consistent with emission models that employ pulsation-driven shocks unless the phasing of such models can be shifted.